This invention relates to the field of devices for protecting electric circuits from the hazards of ground fault currents, as distinguished from other types of faults such as short circuits and other overcurrent abnormalities. A ground fault is one which occurs, in a circuit having a neutral conductor grounded at its source, when contact is made with an electrified conductor in the circuit through a person or a conductive article in contact with the ground. This results in a ground fault current flowing back to the grounded source partially or wholly through a ground path.
Ground fault interrupting mechanisms have heretofore been incorporated in devices such as circuits breakers installed in a permanent branch circuit for indefinite continuous protection, and in portable units for temporary field use to protect portable tools and the like which can be readily connected and disconnected to the available power source at the location of use. These are typically large devices because of the number and size of components involved.
Ground fault sensing devices generally utilize a differential transformer consisting of a frame or ring type core surrounding a central opening or "window". The line or electrified conductor and the neutral conductor of the protected circuit may be wound on the core, or merely pass through the "window" whereby they constitute a single turn primary winding. Current to and from the load through these conductors produces mutually cancelling magnetic flux resulting in zero magnetic flux in the transformer core. However, when a portion of the current flows to ground because of a ground fault and returns to source through a ground path rather than through the neutral conductor, the magnetic flux resulting from current flow through the conductors becomes unbalanced and does not cancel out. A voltage or fault signal is thereby induced in a secondary winding provided on the core of the transformer, this winding being electrically associated with circuit interrupting means for opening the protected circuit. Ground fault interrupters can be made responsive to fault currents as low as 4 or 5 milliamperes, thereby adding substantially to protection of human life as well as protection from other hazards resulting from ground faults.
It is desirable to make such protection readily and conveniently available for general household use, particularly in areas most susceptible to ground faults such as bathrooms, kitchens, swimming pools, garages, and in other areas such as construction sites. It is therefore advantageous to provide a receptacle which incorporates ground fault protection therein and which is still small enough to fit in ordinary household or common trade size junction boxes. Ground fault protection can thereby be made available easily and inexpensively for existing circuit installations merely by replacing an existing receptacle with one incorporating ground fault protection in accordance with this invention. One modification includes feed-through protection, so all outlets in the circuit on the downstream side of the feed-through ground fault receptacle, i.e. opposite the source side, are provided with protection against ground fault hazards. Miniaturization of the ground fault interrupting circuitry and mechanism is required in order to incorporate a complete system within a receptacle small enough to fit ordinary household or common trade size junction boxes. More efficient use of space is also required, and achieved by such measures as employing a circuit scheme that eliminates the need for one or more components which are necessary in other presently known ground fault protection devices, and by making certain components perform more than one function.
The invention herein accomplishes the objectives of miniaturization and more efficient use of space, enabling incorporation of a complete ground fault interrupter within a small receptacle that can be mounted in an ordinary size household type junction box. Such receptacle in accordance with this invention includes additional features such as (a) structural embodiments which qualify for 20 ampere rating in 120 VAC circuits, (b) an anti-teasing member to prevent the reset button from "teasing" open or separating the contacts to a less than fully open position, (c) inclusion of a ground terminal having a ground attachment screw, (d) clamp plate type terminations, (e) heavy duty NEMA type plug terminals, (f) feed-through protection of downstream outlets, and (g) an audible signal means to provide a warning if the receptacle is back wired.